Such a method is intended in particular to install the elongate element in a large stretch of water, such as anchor piles for a fluid-operated device in a stretch of water, or other components of such an installation.
The anchor piles are inserted at the bottom of the stretch of water. They are intended, for example, to retain offshore structures, such as semi-submersible platforms, of the floating-type column type (“SPAR”), or the cable-tensioned type (“Tension Leg Platform” or “TLP”) or floating installations of the FPSO type. Alternatively, the offshore structure is a fixed platform such as a “jack up platform” or any anchor element on the seabed.
To ensure retention of an offshore structure, each elongate element has a substantial length in a horizontal position and therefore a significant height when it is in a vertical position. The horizontal length is, for example, longer than 10 m and in particular in the order of 50 m to 150 m.
The elongate element has, for example, a diameter greater than 1 m, and especially greater than 5 m.
When the elongate element is made of base metal, it therefore has a significant weight of several hundred tonnes.
Therefore, the elongate element cannot simply be manoeuvred using a crane, since the crane would have to have too great a height and lifting capacity to be mounted on a surface laying facility, such as a laying vessel.
In a known installation method, each elongate element is arranged on a barge in a horizontal position, and is brought to the place of installation by the barge.
At the installation location, the barge is arranged alongside a laying vessel. Then, connecting members are mounted on the elongate element, at a distance from the ends. Each connecting member comprises a sleeve arranged around the elongate element and an axle inserted through the elongate element by means of holes made in the wall of this element.
The sleeve and the elongate element are rotatably mounted on the axle. When the elongate element is to be immersed, a first connecting member is rotatably fixed to a support trellis projecting laterally with respect to the laying vessel.
Then, the other connecting member is released. The elongate element then pivots around the axle of the first connecting member under the effect of its own weight in order to adopt a vertical configuration.
Then, a lowering line is attached to the upper end of the elongate element. The connecting member fixed on the trellis is then released to lower the elongate element in the stretch of water by unwinding the lowering line.
Such an approach makes it possible to install elongate elements of a substantial size. However, it is not entirely satisfactory.
In fact, the installation operations to be implemented for the method are relatively complex, since it is necessary to make holes in the wall of the elongate element, and then mount an axle through the holes.
It is also difficult to remove the connecting member once the elongate element is at the bottom of the water.
Moreover, making holes through the elongate element may, in some cases, reduce its mechanical strength and therefore weaken the elongate element.